Difference between a paintmap and wetmap
When a brush with a Color and a Wetness moves through a canvas, it lays paint. 

As a result, the canvas gets 


a color, represented by the (RGBA) Paintmap, and affected, for instance, by the Dissolve effect



(no dissolve in this example)
some wetness, represented by the (greyscale) Wetmap, and affected, for instance, by the Dry effect.




Once the paint dries out (if it does), its color remains while its wetness goes away.

Example shader where the color is the paintmap, the specularity is the wetmap, the roughness is one minus the wetmap (there is also some alpha-based bump here):